The present invention relates to a novel method for measuring the absolute light throughput of polarization-modulating reflective display systems in a given optical system.
An optical system is defined in the present case as a system that prepares light to achieve a desired optical image. The term optical system is intended to include display systems such as head-mounted displays and electronic projection displays.
Every optical component in an optical system has a light throughput or efficiency. Light throughput of a transmissive optical component is the fractional amount of the incident light that the component transmits, i.e., the percentage of light that is not reflected, absorbed or scattered by the component. The light throughput of a reflective optical component is defined as the fractional amount of the incident light that the component reflects, i.e., the percentage of light that is not transmitted, absorbed or scattered by the component.
Light output is a significant factor in assessing and designing optical display systems, such as electronic projectors. In meetings and presentation, presenters frequently use electronic projectors coupled to personal computers to deliver electronic presentations (e.g., Microsoft PowerPointTM presentations). The light output of the system helps determine the brightness of the projected images, so high light output is desirable. Light output is affected by each component in direct proportion to the throughput of that component.
Optical display systems generally use either transmissive mode or reflective mode displays. In transmissive mode systems the light passes through the display or imager. In reflective mode systems, the display reflects light.
Many companies are concentrating on developing reflective-mode displays seeking lower costs and higher resolutions. Promising new reflective-mode displays include liquid crystal on silicon (LCOS) devices. These devices use polarized incident light and reflect polarization-modulated light.
Designers of reflective-mode displays traditionally use a model or measurement of the light transmitted through each component in the system, assigning a fractional transmission value, and then calculating the product of all these transmission values (see, E. H. Stupp, M. S. Brennesholtz, Projection Displays, Wiley-SID Series in Display Technology, 239 ff. (1999), F. E. Doany, R. N. Singh, A. E. Rosenbluth, and G. L.-T. Chiu, xe2x80x9cProjection display throughput: Efficiency of optical transmission and light-source collection,xe2x80x9d IBM J. Res. Develop., 42, 387-399(1998), which are hereby included by reference). To date, it has been difficult to establish such fractional transmission levels for liquid crystal on silicon (LCOS) microdisplays, or even to compare throughput specifications of one LCOS device with another. This is due to the difficulties of accurately modeling or measuring throughput for these devices.
Measuring the throughput is difficult due to the reflective nature of the displays. Because they are reflective, the throughput of an LCOS microdisplay is generally measured as compared to a reference, usually a quarter-wave film (QWF) laminated to a mirror. A traditional method for calculating reflective display output in a system first measures the light throughput of the system using a quarter-wave plate and a mirror in place of the reflective display. J.H. Morrissy et al., xe2x80x9cReflective Microdisplays for Projection or Virtual-View Applicationsxe2x80x9d, 808, SID 1999 Digest, describes a procedure where the reflectivity of reflective-mode displays is calibrated using an aluminum mirror optically coupled to a broad-band quarter-wave retarder plate, or QWF. In this technique, an imager is first driven to its fully bright state and the resulting light output is measured. Then the imager is replaced with the quarter-wave laminate described above and the output is measured again. The first measurement is divided by the second measurement to provide a throughput value.
While such measurements can yield good comparative results between various LCOS microdisplays, absolute microdisplay throughput depends in this case on knowing the reflectance of the reference mirror and QWF. This is especially difficult when spectral throughput is desired. Problems with this approach include the fact that both the mirror and the QWF introduce unknown inaccuracies into the measurement. Since there are no perfect optical devices, both the mirror and the QWF have their own throughputs, which are ignored in the calculation. There are no standard mirrors or quarter-wave plates used for these measurements, so measurements for the same displays are not uniform. As a result, the tester then is faced with the task of trying to define the optical characteristics of the mirror and quarter wave plate (see, e.g., M. D. Wilson, xe2x80x9cMethods of Measuring Performance of LCOS Microdisplaysxe2x80x9d, Microdisplay 2000, Aug. 7-9, 2000).
Furthermore, every display may behave differently in a different system. For example, quarter-wave plates are traditionally only precisely one quarter-wave at one wavelength, so each test has to use at least three different quarter-wave plate/film for red, green, and blue (xe2x80x9cRGBxe2x80x9d, the primary colors used to produce every other color in traditional display systems). Different plates and different RGB tests will yield different results. Even a broadband QWF has different transmission rates for each RGB color. Other factors such as the incidence angle, the temperature, the lamp spectrum, and the input and output#s of the particular optical system also affect the measurements.
The need remains for a method to calculate the absolute throughput of reflective displays in a system in an accurate and repeatable fashion.
The present invention is directed to a method for measuring the absolute light throughput of a polarization modulating reflective display in an optical system having a folded light path. The method includes the steps of measuring a first light intensity, LR, delivered in the folded light path. The folded light path includes an illumination system producing a light beam, a first polarizing beam splitter, the reflective display, and a projection system. A first polarization component of a light beam prepared by the optical system is folded by a first polarizing beam splitter and a second polarization component of the light beam is transmitted by the first polarizing beam splitter, wherein one of the polarization components is reflected off the reflective display.
A second light intensity, LO, delivered by the optical system in an unfolded light path also is measured. The unfolded light path includes the same components as the folded light path, with the exception that in the unfolded light path, the reflective display has been removed. The unfolded light path has the first polarizing beam splitter and a second cross rotated polarizing beam splitter having equivalent optical performance characteristics. The light beam is transmitted by one of the PBS and reflected by the other.
The absolute throughput, TM, then is calculated, where TM=LR/LO. 
The polarization component reflected off the reflective display may be either the first or the second polarization component. The steps of measuring the first and second light intensities may be accomplished photopically, radiometrically, spectro radiometrically or by other suitable methods.
The illumination system may have variable f/#s, the method further including repeating the steps of measuring the first and the second light intensity for different f/#s of illumination and calculating the absolute throughput for the reflective display for different f/#s of illumination. The projection system also may have variable f/#s, the method further including repeating the steps of measuring the first and the second light intensity for different projection f/#s and calculating the absolute throughput for the reflective display for different projection f/#s.
The illumination system includes a light beam source. The beam of light may include visible light, infrared radiation, ultraviolet photolit patterns, or other types of radiation to be imaged using the reflective display. The light beam source may be a coherent light beam source, a collimated light beam source, or other suitable light sources.